Just my two cents worth...
I don't see a way that you could possibly do this within Zen Cart.
You won't know if the address is confirmed or not before you send the customer off to Paypal to complete the transaction - so how are you going to show the charge on the order confirmation page, and how do you include it in the order total that goes with the customer to Paypal?
If your Paypal IPN response comes back with an unverified address status, are you then going to ask the customer to go back to Paypal and pay an extra charge before you process the order? Sounds like a great way to lose customers....